Wellbore isolation during stimulation (for example by fracturing, acidizing, and acid fracturing) is performed by a variety of methods within the oilfield industry. One of the approaches involves the use of ball sealers, which are meant to seal the perforations and prevent fluid in the wellbore from flowing through the perforations into the formation.
Ball sealers are typically spheres designed to seal perforations that are capable of accepting fluid, and thus divert reservoir treatments to other portions of a target zone. Ball sealers are slightly larger than the perforations and are incorporated in the treatment fluid and pumped with it. They are carried to the perforations by the fluid flow, seat in the holes, and are held there by differential pressure. The effectiveness of this type of mechanical diversion requires keeping the balls in place and completely blocking the perforations, and depends on factors such as the differential pressure across the perforation, the geometry of the perforation, and physical characteristics of the ball sealers.
Ball sealers are made in a variety of diameters, densities, and compositions, to adjust for different wellbore conditions and for perforation size. They may be either soluble or non-soluble. Soluble ball sealers are most commonly made of one soluble component, while non-soluble ball sealers often consist of a rigid core surrounded by a rubber (or other material) coating. The shortcoming of either ball sealer type lies in the relationship of the shape and composition of the ball sealer and the shape of the entry hole in the casing. Due to the nature of shooting perforations into casings, one obtains burrs and uneven surfaces that are difficult to seal with a smooth and/or spherical ball. In addition, an elongation of the entry hole may occur due to the casing curvature and the gun orientation when shooting perforations with a non-centralized perforating gun.
There is a need for improving the ability of ball sealers to close off perforations completely. This invention provides such a method involving pumping suitable particles, for example fibers, that plug the small flow paths that may otherwise remain in the perforations around the seated ball sealers.